The mechanisms by which the F plasmid can become integrated into the E. coli chromosome are being studied. Many integration events for F occur by recombination via one of its IS elements at corresponding chromosomally-located IS2 or IS3 elements. Physical studies have indicated specificity in the sense that some IS elements are more active in F integration than are others. The genetics of these processes are being investigated, with recombination products being monitored by physical means. The basis for differences in recombinational activities of specific IS3 elements are being sought by physical and chemical analysis.